1. Field of the Invention
The present invention relates to a semiconductor light-emitting device.
2. Description of the Related Art
These days, development of semiconductor lasers has remarkably progressed so as to be applied to the light source of the optical data processing device such as an optical disk system, or high-speed laser printer. As the density of the optical disk increases, the demand of the laser having a shorter oscillating wavelength is being raised, and therefore the development of this field is greatly promoted.
A conventional semiconductor laser prepared by forming an InGaAlP semiconductor layer on a GaAs substrate, has an oscillating wavelength of 0.6 .mu.m, and a laser output of about 20 mW, and is already used in practice. When the semiconductor laser of this type is used as a light source of an optical disk system, the recording density is improved by only 1.4 times as compared to the case of a semiconductor laser having GaAlAs semiconductor layer. Such a laser is not sufficient as a light source used for a high-density recording operation.
A semiconductor laser made of a compound semiconductor containing elements from the group II and group VI of the periodic table such as ZnSe has an oscillating wavelength of about 0.5 .mu.m, and is able to emit blue light. Further, in the case where the semiconductor laser is used as a light source of an optical disk system, the recording density is increased by 3 times as compared to the case where a GaAlAs semiconductor laser is used. Thus, the development of semiconductor laser by use of a ZnSe compound semiconductor is greatly promoted.
In general, a p-type ZnSe compound semiconductor entails a drawback that a good ohmic contact cannot be obtained between ZnSe and a metal electrode. In order to solve this drawback, there is a technique in which a contact layer made of, for example, GaAs, is provided between the ZnSe layer and the metal electrode. However, the semiconductor laser having the above-described structure involves a large band gap difference between the energy of ZnSe, 2.7 ev, and that of GaAs, 1.4 eV. Such a large band gap difference causes a hetero-spike in the valence band, which acts as a barrier for injected holes. As a result, a significant voltage drop occurs on the hetero contact interface between ZnSe and GaAs, thereby excessively increasing the operating voltage.